A 10 min read

In this article, we’ll look at the solution to this Official Guide (OG) question CR02531.01 – “A Fossil Recently Discovered” on Critical Reasoning:

CR02531.01- A fossil recently discovered in Marlandia, a chain of islands, proves that a present-day reptile indigenous to Marlandia is descended from an ancient reptile species that lived on the islands millions of years ago. The finding is surprising since the ancestral species was thought to have become extinct when Marlandia was submerged in a global sea-level rise twenty-five million years ago. Based on the new discovery, many scientists have concluded that the sea-level rise in question left at least part of Marlandia unsubmerged.

Which of the following would, if true, provide the most additional support for the scientists’ conclusion?

[Refer to GMAT Official Guide for Options]

Here is some general information about this official guide question:

OG Video Solution – CR02531.01 – A Fossil Recently discovered… | “Marlandia”

OG Solution – CR02531.01 – A Fossil Recently discovered… | “Marlandia”

“If you don’t ask the right questions, you don’t get the right answers.”

– Edward Hodnett (author of The Art of Problem Solving; How to Improve Your Methods)

“Ask the right Prethinking question, get the right answer”

– Your friendly neighborhood CR expert at e-GMAT

Hey folks,

In this article, we are going to analyze a CR question from OG Advanced. This question is classified as “Hard” but is fairly easy as per me – especially if you follow the sage advice above 😊.

Extracting the logic from the argument

What the argument tells us:

1. Marlandia is a chain of islands.
2. A fossil was discovered there recently.
3. There is a present-day reptile that is native (i.e., indigenous/originally from) to Marlandia – let’s call it X.
4. There used to be an ancient reptile species that lived in Marlandia millions of years ago – let’s call it Y.
5. FACT: the fossil is proof that X (the modern-day reptile) descended from Y (the ancient reptile).
   1. “Descended from” – means there is a direct line of reptiles from the ancient Y to the present-day X.
   1. Inference – this means that the ancestor, Y, never went extinct. It survived and evolved over time to become X.
      1. Why – If the line got broken, then we cannot say that X is a descendent of Y. To be a descendent, there must be a continuous, unbroken line of the species from Y to X.
6. As per the author, the above finding is surprising:
   1. Why: because Y was thought to have become extinct.
      1. When did this supposed extinction occur: When Marlandia was submerged in a global rise of sea-level 25 million years ago.
7. Based on the new discovery (fossil),
   1. Scientists’ Conclusion: The sea level rise that happened 25 million years ago left at least part of Marlandia unsubmerged.

The question asks us to strengthen the scientists’ conclusion.

At this point, we have a solid understanding of the information given in the argument, but to strengthen the conclusion, we need to extract the underlying logic used to arrive at the conclusion.

On what basis does the scientists’ claim that the sea-level rise that happened 25 million years ago left at least part of Marlandia unsubmerged?

At the end of the day, this claim hinges on one point – the fact that the ancestor Y survived the rise in sea-level that occurred 25 million years ago and did not go extinct at this time (as was earlier believed).

The scientist believes that the reason Y survived the sea-level rise 25 million years ago was that at least some small part of Marlandia remained unsubmerged. The implication here is that the ancestor Y managed to survive at that point in time by staying afloat on the unsubmerged piece of land.

To summarize,

The Conclusion: the sea-level rise that happened 25 million years ago left at least part of Marlandia unsubmerged.

The Logic: the ancestor Y survived through this sea-level rise; this must have happened on account of Y staying afloat on the unsubmerged piece of land.

Prethinking

Prethinking – Arriving at the correct question

Now that we have distilled the real logic from the argument, let’s first identify those scenarios where the conclusion will break. This is my favorite method of coming up with strengtheners – arriving at conclusion-breakdown scenarios (also known as falsification conditions), and then forming statements that guard against them. Let us see this in action.

When can the scientist not conclude that the sea-level rise that happened 25 million years ago left at least part of Marlandia unsubmerged?

Given that the ancestor Y survived through this sea-level rise.

The logical answer – if the real reason the ancestor Y survived the sea-level rise 25 million years ago was not that it stayed afloat on a supposedly unsubmerged piece of Marlandian land, but something else.

So, the more important question to ask here is –

What else can explain how the ancestor Y survived the sea-level rise 25 million years ago?

Any valid answer to the above question will break down the conclusion. Any valid answer to this question is a valid falsification scenario.

Prethinking – Arriving at strengtheners

So, let’s assume that every part of Marlandia got submerged in the sea-level rise 25 million years ago.

What else can explain how the ancestor Y survived the sea-level rise 25 million years ago?

Here are a few situations I could think of:

1. What if the ancestor Y could survive in water as well, not just on land?

If Y had the ability to survive in water 25 million years ago, then it can explain how it survived despite Marlandia getting submerged in water completely at that time.

Strengthener: The ancestor reptile (Y) could not have survived in water.

Why does this work? If Y could not have survived on water, then it increases our belief in the notion that had Marlandia been completely submerged, the species would have gone extinct. Now, given that Y did not actually go extinct, this increases our belief in the conclusion that at least some part of Marlandia would have remained unsubmerged.

• What if the ancestor Y could have migrated by flying to another landmass when Marlandia got submerged and could have come back to the island later?

If Y could fly away to a safer land to avoid getting submerged 25 million years ago, and then come back to Marlandia at a later time, this could also explain how Y survived despite Marlandia getting submerged in water completely at that time.

Underlying Strengthener: The reptile ancestor Y could not have flown away to a different land to avoid getting submerged by the sea-level rise and come back to Marlandia at a later point in time (when Marlandia was not still submerged).

Here is another creative strengthener, also involving the idea of migration, but on land 😊.

• What if Y could have avoided getting submerged in Marlandia during the sea-level rise by migrating to a connected piece of landmass (that was not submerged), and then migrated back to Marlandia at a later point in time when the island was not submerged anymore?

And at some point of time after this reverse migration, Marlandia broke off from the landmass.

If this occurred, it could also explain how Y survived despite Marlandia getting submerged in water completely at that time. Y simply migrated away to the connected landmass and then came back to Marlandia later.

Strengthener: During the time of the rise in sea-level, the ancestor reptile could not have survived by migrating by land to a then-connected landmass (that was not submerged), and then migrating back to Marlandia at a later point in time when the island was not submerged anymore (before the island broke off from the landmass).

There are other possible situations (and so, other strengtheners) too, not just these three.

Bottom Line – if you ask the correct question, prethinking becomes easy (and fun 😊).

Answer Choice Analysis – arriving at the correct answer

Let us now analyze the answer choices.

Option A:

Even if reptiles in Marlandia adapted to several environmental changes since the sea level rose 25 million years ago, it tells us nothing about whether at least some part of Marlandia remained unsubmerged when this rise in sea level occurred.

Why?
As per option A, the reptiles in Marlandia have adapted to the environmental changes (for example – a rise in temperature) after the environmental change that is the rise in sea level. But this does not tell us whether the ancestor species Y was able to adapt to the environmental change that was the rise in sea level, which submerged Marlandia.

If Y adapted to survive in Marlandia even when it was submerged (in other words, if Y adapted to live on water/underwater), then it reduces our belief in the conclusion that at least some part of Marlandia remained unsubmerged. If, on the other hand, Y could not adapt to survive on water/underwater, then it increases our belief in the conclusion – the fact that Y still survived without adapting to survive on water must mean that some part of Marlandia remained unsubmerged.

But option A only talks about reptiles adapting to environmental changes since the rise in sea-level. It does not tell us anything about whether this specific ancestor Y adapted to survive on water when Marlandia got submerged due to the rise in sea level. So, it does not strengthen the conclusion that at least some part of Marlandia remained unsubmerged when this rise in sea level occurred.

Option B:

Here is an example to better understand the implications of option B.

Sri Lanka broke away from the Indian subcontinent many years ago.

Does this statement in any way increase our belief that at least some parts of Sri Lanka will get submerged/not get submerged in the future, as water levels rise?

Nope!

Now, let’s look at option B more closely.

Event 1: Marlandia separated from a much larger landmass 80 million years ago.

Event 2: At least some part of Marlandia did not get submerged when the sea-level rose 25 million years ago.

Does event 1 increase your belief in event 2?

No. A statement telling us that Marlandia broke off from a larger landmass 80 million years ago tells us nothing about whether some part of Marlandia stayed unsubmerged when the sea-level rose 25 million years ago.

A question: Does option B not guard against a conclusion-breakdown scenario and therefore, strengthen the argument?

Not exactly. This option is somewhat related to our prethought strengthener 3 in that it explores the connected landmass angle, but it falls short of being a strengthener.

Issue with choice B:

The issue: There could be another landmass to which Marlandia was still connected 25 million years ago. The fact that one landmass got disconnected 80 million years ago does not in any way prevent the existence of another connected landmass that stayed unsubmerged during the sea-level rise.

In other words, option B is not really guarding against the conclusion-breakdown scenario we discussed earlier. Hence, this is not a valid strengthener.

Observe the difference in wording between our strengthener 3 and option B. Option B keeps open the possibility of some other connected landmass, whereas our prethought strengthener rules out the possibility of the ancestor Y migrating to any such connected landmass.

The lesson: An answer choice can be related to our prethought idea, but still be worded in a way that makes it incorrect. So, do not rush with your analysis of the answer choice because it is aligned to your prethought idea; analyze its meaning and implications deeply and carefully.

Option C:

This is the most popular incorrect choice. And with good reason – it is close. Let’s analyze this!

What option C means: No fossils proving that X descended from Y have been found anywhere except Marlandia.

Remember – the fossil discovered is of the ancestor Y, known to have inhabited Marlandia 25 million years ago.

This option is trying to suggest that the ancestor Y was not living in any other place (apart from Marlandia) 25 million years ago.

If Y did not live anywhere else apart from Marlandia, it removes the possibility

• that the species survived because Y lived in some other place that did not get submerged, not just Marlandia, AND
• some members (at least) of the species moved to Marlandia at a later point in time, when it was not submerged.

In other words, it can increase the belief that at least some part of Marlandia did not get submerged, by removing an alternate explanation for the survival of Y.

But here is the problem. Option C only tells us that no such fossils have been found anywhere other than Marlandia.

The reason why no such fossils have been found can be either of the following:

1. No such fossils even exist.

2. Such fossils definitely exist. It is only that they have not yet been found.

The issue with option C: we cannot say with any level of certainty whether the reason why no such fossils have been found so far is point 1 or point 2. We cannot even say that the likely reason is that no such fossils exist – the real reason could very well be point 2.

Option C only tells us that no such fossils have been found. Unless we know that no such fossils exist, option C cannot be considered a strengthener.

Option D:

While the present-day reptiles, including X, may indeed be able to survive and thrive on even the tiniest patch of land, this does not tell us anything about the ancestor Y.

The argument is only concerned with the ancestral species Y – how did it survive? Did it survive by surviving on a tiny patch of unsubmerged island, or did it survive some other way?

The option does not address these questions.

For instance, what if Y could not have survived on the very tiny islands? Then, it reduces our belief that they would have survived by staying afloat on some portion of Marlandia that stayed unsubmerged. This would weaken our conclusion.

But option D does not give us any information about the ancestor Y; it only talks about present-day reptiles. It is irrelevant.

Option E:

This is the correct answer. We can see that this is in line with our prethought strengthener 1.

As discussed in prethinking, If Y could not have survived at sea, then it increases our belief in the notion that had Marlandia been completely submerged, the species would have gone extinct. Now, given that Y did not actually go extinct, this increases our belief in the conclusion that at least some part of Marlandia would have remained unsubmerged. Hence, option E is a definite strengthener.

Takeaway – CR02531.01 – A Fossil Recently…

1. Do not just try to simply understand the meaning conveyed by the sentences in an argument. Seek to extract the logic which is used to arrive at the conclusion.
2. Once you have extracted the logic, if you ask the right question, you can get several right answers by sharp prethinking.
3. Analyze options deeply – do not rush, understand both the meaning and the logical implications.
4. Lesson from option B – An answer choice can be related to our prethought idea, but still be worded in a way that makes it incorrect.
5. Lesson from option C – if no evidence of X has been found so far, it can mean either that:
   1. No such evidence exists.
   1. The evidence exists, only that it has not been found yet.

We cannot say with any level of certainty or likelihood that no such evidence exists, based on the statement that no such evidence has been found. There you go! I hope that you enjoyed reading this article as much as I enjoyed writing it.

If you want to solve more questions that give you complex answer choices, try these out:

• Suriland
• Diatonic Scale

Happy learning!
Harsha

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